1. Field of the Invention
The invention relates to a process for the synthesis of a heat-curable silicone resin which is soluble in organic solvents, by partially reacting an alkoxysiloxane with an at least difunctional, low molecular weight polyol, especially in the presence of a catalyst.
2. Description of the Prior Art
German Offenlegungsschrift No. 28 28 990 discloses a process for synthesizing heat-curable silicone resins by reacting alkoxysiloxanes with polyfunctional, organic hydroxyl compounds. In this process, siloxanes, having the general formula ##EQU1## in which OR.sup.1 is an alkoxy group of primary or secondary alcohols with 1 to 4 carbon atoms,
R.sup.2 is an alkyl and/or phenyl group, PA1 a has a value of 1.0 to 1.5 and PA1 b has a value of 0.1 to 0.7, PA1 R.sup.2 is an alkyl or phenyl group, PA1 a=1.0 to 1.2, and PA1 b=0.5 to 1.2,
are reacted with low molecular weight, multihydric alcohols at temperatures of 100.degree. to 160.degree. C. Optionally, the reaction is carried out in the presence of known transesterification catalysts and solvents. The alcohol R.sup.1 OH is removed in a quantitative ratio, such that one SiOR.sup.1 -group corresponds approximately to one COH-group and the reaction is terminated at a degree of conversion of 25 to 80% by cooling to a temperature below 100.degree. C.
In this reaction, the OR.sup.1 -groups are methoxy, ethoxy, n-propoxy or isopropoxy or n-butoxy groups. The low molecular weight multihydric alcohols used can be ethylene glycol, trimethylolethane, trimethylolpropane, neopentyl glycol, glycerin, pentaerythritol, dimethylolcyclohexane as well as mixtures thereof.
The cured silicone resins prepared by this process have a very high silicone content of 63 to almost 98% by weight and therefore, have high thermal stability and good elasticity. These products can also be processed in the presence of relatively small amounts of solvents. However, because of their high silicone content, these products are thermoplastic, so that they cannot be used for baked enamels which have to be stable at high temperatures, such as, for example, decorative coatings on household utensils.
Suggestions have been made for combining the desirable enamel properties of polyester resins, especially their high surface hardness, low thermoplasticity and good chemical resistance, with the good properties of the silicone resins, especially their high thermal stability.
U.S. Pat. No. 3,044,979 discloses silicon-containing polyesters which are synthesized from 70 to 95 weight percent of a mixed hydrolysate of chloro-organosilanes with 1.0 to 1.75 organic groups per silicon atom and 30 to 5 weight percent of a polyester with a viscosity of 38,000 to 42,000 cP, which consists of lower alkyl esters of isophthalic acid, terephthalic acid or their mixtures, and ethylene glycol and a saturated, multihydric alcohol with at least 3 OH-groups in the molecule. The products also have a very high thermoplasticity, especially at high silicone contents. In addition, a non-volatiles content of more than 50% in the vehicle solution is difficult to obtain in practice because of the high viscosities.
Silicone resins, modified with polyesters and having a high flexibility and swedging capability, are described in German Auslegeschrift No. 22 32 419. In the process disclosed therein, saturated polyesters are used with a hydroxyl number of 60 to 350 and polysiloxane blocks, which have 0.25 to 0.70 moles of SiOR-groups per 100 g, in a molar ratio of 1.25 to 0.8 relative to the reactive groups. However, the thermostability of these resins is inadequate at silicone contents below 50 percent. It is a serious disadvantage of these modified silicone resins that they require considerable amounts of solvent in order to adjust the viscosity to values required for processing.
German Offenlegungsschrift No. 25 21 170 discloses silicone/polyester copolymers, which are intended to be particularly useful for coating applications. The process for their preparation is characterized by heating a reaction mixture of a silane hydrolysate and a polyester, rich in hydroxyl groups, in the presence of a glycol ether acylate of a carboxylic acid. It is stated in this Offenlegungsschrift that the product of this process generally consist of about 70 to 95% silicone, the rest being polyester. A copolymer which contains, for example, 85.5% silicone and 12.5% polyester, is most preferred. With such a composition, these products are not very suitable for thermostable, decorative enamel finishes because of their high thermoplasticity. It is furthermore pointed out in German Offenlegungsschrift No. 25 21 170 that, in contrast to the process of U.S. Pat. No. 3,044,980, in which the polyester is used without solvents in the copolymerization, solvents from the group of glycol ether acylates can be used to advantage. In this connection, a solids content of about 55 to 65% is recommended. For this reason, these products are also not suitable for coatings, which save energy and raw materials and which are compatible with the environment.
Finally, German Offenlegungsschrift No. 29 49 725 discloses a process for synthesizing polyesters modified with organo(poly)siloxanes by reacting branched organo(poly)siloxanes, containing silicon-linked alkoxy groups with polyesters, containing alcoholic hydroxyl groups. The polyesters are produced from carboxylic acids and/or their anhydrides and or their esters, which contain at least two carboxyl groups per molecule and are free of aliphatic multiple bonds, and alkanols and alcohol, containing 1 to 4 carbon atoms and having at least two hydroxyl groups per molecule as the only reactants used for producing the polyester. Characteristically, an organo(poly)siloxane is used as the branched organo(poly)siloxane containing silicon-linked alkoxy groups at least 95% of whose SiC-linked organic radicals consist of methyl radicals, of which only one radical in each case is linked to a silicon atom. The alkoxy groups of the siloxane consist of methoxy groups, ethoxy groups, or mixtures of these alkoxy groups, and contain at most 0.5 weight percent of silicon-linked hydroxyl groups and, after ashing, at least 40 weight percent of silica based on the weight of the respective organo(poly)siloxane before ashing. These alkoxysilanes are reacted with conventional, saturated polyesters having hydroxyl groups. The polyesters used in this reaction, are of such high molecular weight, that they must be diluted to a solids content of about 60%, in order to adjust the viscosity to a value suitable for the reaction with the alkoxysiloxane. The products of the process are suitable for coating, for example, wood, organopolysiloxane elastomers, or aluminum, and as an adhesive for mica, glass fiber, rock wool, magnesium oxide or silica. The products are especially useful in the manufacture of electrically insulating materials, but not for producing coatings which are resistant to thermal stresses.
Thus, the reaction between alkoxy-functional organosiloxanes and polyesters, containing hydroxyl groups, is well known. Some of the products obtained can be used as enamels. In this connection, however, the modified silicone resins generally have such a high viscosity, that it is not possible to exceed solids contents of 60% by significant amounts.